Title: The nature and origin of substructure in the outskirts of M31

Abstract:
Wide-field surveys of the M31 halo have revealed much substructure in the form of streams, 
clumps, and shelves. However, understanding the nature and origin of this material is not 
straightforward from morphology alone. I will present further constraints in the form of 
quantitative star formation histories for 10 deep HST/ACS fields in the outskirts of M31. Half 
of these fields are dominated by material stripped from the Giant Stellar Stream (GSS), and are 
characterised by an age--metallicity relation showing rapid chemical enrichment to solar 
metallicity by z=1, suggestive of an early-type progenitor. The other fields have stellar 
populations similar to those of the thin disc; most of their mass has formed since z~1. Finally, a 
significant number of 2 Gyr old stars produced in a disc-wide burst are found in all our fields 
regardless of their location. This suggests that they have not formed in situ but have been 
kicked-out from the thin disc into the halo through disc heating in the recent past, likely a 
consequence of the accretion of the GSS progenitor.